It is known that olefinically unsaturated compounds having two conjugated carbon-carbon double bonds can be catalytically carbonylated with carbon monoxide and an alcohol to yield carboxylic esters. However, the known processes have various drawbacks which render them rather unattractive for use on a commercial scale.
A process for the preparation of carboxylic esters or carboxylic acids by reacting 1,3-butadiene with carbon monoxide and an alcohol or water and in the presence of a palladium catalyst is described in U.S. Pat. No. 3,501,518. The palladium catalyst is prepared by combining (a) palladium metal or a palladium chalcogenide, (b) an organic phosphine, for example a triarylphosphine and (c) an acid, for example hydrogen chloride. The palladium metal or palladium chalcogenide is present as a solid phase dispersed in the reaction mixture. Example 15 of the patent shows that a very high pressure of carbon monoxide was used and that, starting from 1,3-butadiene, no carboxylic di-esters had been formed.
According to U.S. Pat. No. 3,437,676 1,3-butadiene can be reacted with carbon monoxide and an alcohol or water in the presence of a palladium catalyst having the formula L.sub.m PdX.sub.n, in which L may be an organic phosphine, X may be chloride, m is an integer from 1 to 4, n is 1 or 2 and m+n is 2 to 6. However, the pressures applied are very high: 709 bar in the two examples concerning the carbonylation of 1,3-butadiene. Moreover, carbonylation of both double bonds has not been observed: in Example 63 of the patent, the reaction mixture contained a monocarboxylic acid as the sole reaction product.
British Patent Specification No. 1,110,405 describes a process for the preparation of carboxylic esters by reacting a conjugated diene with carbon monoxide and an alcohol in the presence of a catalyst comprising palladium, an organic phosphine and halide ions. It is preferred slightly to acidify the reaction mixture, for example, by providing for the presence of toluenesulfonic acid therein. According to Example 3 of the specification, 1,3-butadiene is converted into a mixture of methyl 3-pentenoate, dimethyl 2-methylglutarate and dimethyl ethylsuccinate in the presence of diiodobis(tributylphosphine)palladium(II) and p-toluenesulfonic acid, but this conversion was carried out at the very high pressure of 1013 bar.
European Patent Application No. 0106379 describes a process for the carbonylation of an olefinically unsaturated compound with carbon monoxide in the presence of water, an alcohol and/or a caboxylic acid, a palladium catalyst, at least 5 mol of a phosphine PR.sup.1 R.sup.2 R.sup.3 in which R.sup.1, R.sup.2 and R.sup.3 each represent an optionally substituted aryl group, per gram atom of palladium and an acid with a pK.sub.a of less than 2, except hydrohalogenic and carboxylic acids. The Applicant has found that an olefinically unsaturated compound having two conjugated carbon-carbon double bonds is hardly converted, if at all, with this known process.
It has now been found that in the carbonylation of olefinically unsaturated compounds having two conjugated carbon-carbon double bonds high yields of carboxylic di-esters can be obtained at a relatively low partial pressure of carbon monoxide by carrying out the reaction in the presence of a catalytic system defined more closely hereinafter.